Wavelength division multiplexed (WDM) optical communication systems are known in which multiple optical signals or channels, each having a different wavelength, are combined onto an optical fiber. Such systems typically include a laser associated with each wavelength, a modulator configured to modulate the optical signal output from the laser, and an optical combiner to combine each of the modulated optical signals. Such components are typically provided at a transmit end of the WDM optical communication system to transmit the optical signals onto the optical fiber. At a receive end of the WDM optical communication system, the optical signals are often separated and converted to corresponding electrical signals that are then processed further.
Preferably, the information carrying capacity of an optical communication system should be optimized to carry a maximum amount of data over a maximum length of optical fiber. In optimizing the capacity, however, certain trade-offs are often made. For example, certain modulation formats may be employed to modulate the optical signals to carry data at higher rates. Such higher rate modulation formats, however, are typically more susceptible to noise, and, therefore, may not be used in transmission of optical signals over relatively long distances.
Capacity may be further increased by transmitting a relatively large number of channels over the optical fiber. Trade-offs, however, are encountered here as well. For example, when increased numbers of channels are provided, each channel is typically provided spectrally close to each other, thereby increasing error rates due to cross-talk, as well as non-linear effects, such as cross-phase modulation (XPM). Moreover, the susceptibility of channel to cross-talk, non-linear effects, and noise are often wavelength dependent. Thus, a channel at one wavelength may have more or fewer errors due to XPM or other non-linear effects compared to a channel at another wavelengths. Accordingly, a maximum capacity may be achieved by optimizing the above noted parameters, such as modulation format, distance, and channel spacing. Such optimized capacity may require non-uniformly spaced channels, for example.
Optical demultiplexers are often employed to separate or demultiplex the combined optical signals. Typically, such optical demultiplexers include optical components that have a fixed bandwidth to select optical signals having a particular wavelength. Accordingly, since different WDM optical communication systems extend over different lengths of fiber, include different types of fiber, and may have other differing characteristics, optical demultiplexers must be tailored for each WDM optical communication system if each such system is to have optimized capacity. As a result, such tailored optical demultiplexers are typically expensive.
Moreover, the wavelengths associated with each optical signal are often uniformly spaced from each other so as to conform to a so-called standardized “grid.” In one such wavelength grid, standardized by the International Telecommunications Union (ITU), wavelengths are spectrally spaced from one another by 50 GHz. Such 50 GHz spaced wavelengths or grid wavelengths include 1569.18 nm, 1568.36 nm, 1567.54 nm, etc. Typically, systems that transmit optical signals having wavelengths conforming to the ITU grid do not transmit optical signals having wavelengths between the grid wavelengths. Thus, such systems may not have a channel spacing or other optimized parameters to provide maximum capacity.
An optical communication system is therefore needed that has flexible channel spacing and bandwidth so that the capacity of such a system can be optimized for a given fiber type and distance, as well as other system parameters.